Class vs ID
Class
.classelement {color:red;}
<div class="classelement">text here<⁄div>
a class can be used multiple times
ID
#IDelement {color:red;}
<div id="IDelement">text here<⁄div>
ID can only be used once
IDs carry a heavier weight in determining styles
IDs can be more specific
This example shows how a links can be defined within an ID only.
<style>
#testid {background:#efefef;;}
#testid a{font-weight:bold;color:navy;text-decoration:none;}
#testid a:hover{color:green;background:yellow;text-decoration:underline;}
<⁄style>
<div id="testid">
<a href="#">Link inside testid ID<⁄a>
<⁄div>
<a href="#">Link outside ID<⁄a>
Link inside testid ID (redefined A link)
Link outside ID (base A link)